Laser Flash Photolysis: Mechanistic and Kinetic Analysis of Radical-Induced Pollutant Degradation

2. Radical Generation and Detection Methods

2.1 Hydroxyl Radicals (HO•)

Hydroxyl radicals (HO•) are classic non-selective oxidative free radicals, with a redox potential of 1.8–2.7 V. They can degrade many organic pollutants, including trace organic contaminants (TrOCs), with second-order rate constants up to ~10⁹ M⁻¹s⁻¹. Because HO• has a very short lifetime (only ~20 ns in aqueous solution), its steady-state concentration is very low (~10⁻¹⁵ M). Additionally, HO• reacts with TrOCs through parallel reaction channels, making it difficult to determine and quantify using conventional techniques. LFP’s high temporal resolution and detection sensitivity provide an ideal method for studying the degradation kinetics of TrOCs induced by HO•.

HO• can be generated through multiple routes. Based on different precursors, it can be classified into two main types. One method involves using N-hydroxypyridin-2-thione (PSH), which undergoes N–O bond cleavage under 355 nm excitation to produce HO• and another radical (PyS•).

Acetonitrile (ACN) is typically selected as the solvent for this reaction environment for the following main reasons:
1. PSH has high solubility in ACN
2. ACN has a very low molar absorption coefficient (ε), so it does not compete with PSH for light absorption
3. ACN has weak reactivity with HO•, thus not interfering with the generation and detection of the target radical

In aqueous systems, HNO₂ and H₂O₂ are common HO• precursors. HNO₂ absorbs in the 320–360 nm range and is commonly photolyzed using 355 nm light.

However, the pH of the aqueous solution has a certain influence on the generation of HO• from the photolysis of HNO₂. When using HNO₂ as a precursor salt, the solution pH needs to be controlled below 1.4. As pH increases, HNO₂ (pKₐ = 3.35) loses a proton to form NO₂⁻. The higher concentration of NO₂⁻ results in it gradually becoming the dominant species during photolysis (see eqns. 3–5). At the same time, NO₂⁻ can also react with the generated HO• (eqn. 6), ultimately causing a significant decrease in HO• concentration in the system, thereby affecting the accuracy of subsequent kinetic measurements.

(3)  NO₂⁻ + hν → [NO₂]⁻*

(4)  [NO₂]⁻* → •NO + O⁻

(5)  O⁻ + H⁺ ⇌ HO•

(6)  NO₂⁻ + HO• → NO₂• + OH⁻

Under neutral or slightly alkaline pH conditions in aqueous solution, H₂O₂ is an ideal precursor for generating HO•. Since H₂O₂ has weak absorption at wavelengths above 280 nm (ε ~ 3 M⁻¹·cm⁻¹), 266 nm laser light is typically used in LFP systems to photolyze H₂O₂. This is currently one of the mainstream methods for generating HO•

 (7)  H₂O₂ + hν → HO• + HO•

HO• detection: Because HO• has very weak absorption in the UV-visible spectral range, it cannot be directly detected. In practice, indirect detection methods are commonly used by introducing a reference compound (RC). The principle is that the RC selectively reacts with HO• to generate a distinct, characteristic absorption signal from the resulting free radical species. In aqueous solutions, SCN⁻ is the most commonly used HO• reference compound. HO• first reacts with SCN⁻ to form the SCNOH• radical. SCNOH• is highly unstable with a short lifetime (~5 ns), and rapidly decomposes into SCN• and HO⁻. The resulting SCN• then reacts with excess SCN⁻ in solution to form the dimer radical anion (SCN)₂⁻•, which has a distinct absorption peak at 470 nm (eqns. 8–9). By measuring the ΔA at 470 nm and using the 1:1 stoichiometric ratio of HO• to (SCN)₂⁻•, the concentration of generated HO• can be calculated.

(8) HO• + SCN⁻ ⇌ SCNOH•⁻
(9) SCNOH•⁻ ⇌ SCN• + OH⁻
(10) SCN• + SCN⁻ ⇌ (SCN)₂•⁻

In organic systems, the commonly used reference compound (RC) is p-nitrosodiphenylamine. The product of its reaction with HO• exhibits a distinct absorption peak at 392 nm. Therefore, by measuring the ΔA of the product at this wavelength, the concentration of HO• can ultimately be determined.

2.2 Sulfate Radicals (SO₄•⁻)

Sulfate radicals (SO₄•⁻) is considered a key transient species in atmospheric and aquatic chemistry. In the atmosphere, SO₄•⁻ mainly originates from the dissociation of sulfate salts in aerosols (SO₄²⁻, HSO₄⁻). In natural waters, SO₄•⁻ can form from the oxidation of sulfur dioxide or via HO•-initiated oxidation of SO₄²⁻/HSO₄⁻. Compared to HO•, SO₄•⁻ has a higher redox potential (2.5–3.1 V) and a longer lifetime (30–40 µs), enhancing its ability to transfer and react with target compounds like TrOCs. Additionally, SO₄•⁻ is more stable over a broader pH range (pH 2–8), making it suitable for water treatment processes. Due to these advantages, sulfate radical-based advanced oxidation technologies have gained wide attention. Currently, common SO₄•⁻ precursors include peroxodisulfate (S₂O₈²⁻, PS) and peroxymonosulfate (HSO₅⁻, PMS). Under UV, ultrasound, or electrochemical activation, these compounds undergo O–O bond cleavage to form SO₄•⁻.

In LFP systems, PS (peroxydisulfate) is a more commonly used SO₄•⁻ precursor than PMS, as PMS photolysis produces SO₄•⁻ and simultaneously generates HO•.

(12) HSO₅⁻ + hν → SO₄•⁻ + HO•

HO• can react rapidly with almost all compounds, making it difficult to study the reaction kinetics of SO₄•⁻ with specific substances. Additionally, PMS is relatively unstable under acidic conditions and may spontaneously produce singlet oxygen (¹O₂). A solution of PS at a certain concentration can generate SO₄•⁻ upon 266 nm laser excitation:

(13) S₂O₈²⁻ + hν → 2SO₄•⁻

SO₄•⁻ has a distinct absorption peak at 450 nm, making it possible to monitor changes in absorbance at 450 nm to observe the kinetics of SO₄•⁻. Figure 2 records the transient absorption spectra of a PS solution (0.01 M) after excitation with 266 nm laser. Clear characteristic absorption peaks were observed at both 330 nm and 450 nm, where the 450 nm absorption is attributed to SO₄•⁻, and the 330 nm absorption is believed to be from PS radical (S₂O₈•⁻), which is generated through the reaction between SO₄•⁻ and PS..

Figure 2: Transient absorption spectra of 0.01 M PS solution after laser excitation. (Image source: Xiao* et al., Water Research. 2023, 224:1, 120526).

2.3 Reactive Chlorine Species (RCS)

During the water treatment process of natural waters and high-concentration chlorine-containing wastewater, common RCS mainly include Cl⁻, Cl₂⁻·, ClOH·/HClOH·, and ClO·. Based on their generation mechanisms, these species are classified as primary and secondary radicals. Primary radicals refer to reactive species generated by directly breaking chemical bonds in compounds (e.g., UV/HClO system generating Cl·), while Cl₂⁻·, ClOH·/HClOH·, and ClO· are considered secondary radicals, derived from reactions of primary radicals and their products. These RCS generally have high oxidation-reduction potentials and thus possess strong reactivity, especially in the degradation of TrOCs (trace organic contaminants), and have great potential in water treatment. This article focuses on the generation and detection of ClOH·/HClOH· and ClO·, mainly for the following reasons:

1. The spectral information of ClOH·/HClOH· is not well-established, making identification difficult.

2. The peak absorption of ClO· is around 280 nm (ε = 890 M⁻¹ cm⁻¹), which is outside the detection range of LFP (Laser Flash Photolysis) instruments (typically 300–900 nm). Therefore, for such secondary chlorine radicals, it is rare to find ideal reference compounds.

In LFP systems, the focus is generally on the generation and detection of Cl· and Cl₂⁻·. Common experimental methods to generate Cl· include direct photolysis of chloroacetone (CA) or carbon tetrachloride (CCl₄):

CH₃COCH₂Cl + hv → CH₃COCH₂· + Cl· (15)
CCl₄ + hv → CCl₃· + Cl· (16)

However, hydrolysis of CA in aqueous solutions may generate Cl⁻, which can further react to form Cl₂⁻·:

CH₃COCH₂Cl + H₂O → CH₃COCH₂OH + Cl⁻ + H⁺ (17)
Cl⁻ + Cl· → Cl₂⁻· (18)

Therefore, when using CA as a precursor for Cl·, the hydrolysis side reaction must be considered. Additionally, over time, due to accumulation of the aldol condensation product (aldol), yellowing of the solution may be observed:

CH₃COCH₂Cl + CH₃COCH₂Cl → aldol (19)

Extra care should be taken during experiments; on-site preparation of CA is recommended. Cl· can also be generated by photolysis of carbon tetrachloride (CCl₄):

CCl₄ + hv → CCl₄· → CCl₃· + Cl· (20)

CCl₄⁻· is the excited state generated upon photolysis. Considering the solubility and quantum yield of the precursor salt, CA is a more efficient precursor for generating Cl· radicals. However, due to the potential for photoinitiated polymerization of CA that could lead to explosion, CCl₄ is generally preferred in experiments for safety reasons.

In most laboratory experiments, Cl₂⁻· is produced through UV/PS/Cl⁻ systems, where Cl⁻ and SO₄⁻· react, and SO₄⁻· undergoes photolysis in the presence of PS to yield:

SO₄⁻· + Cl⁻ → SO₄²⁻ + Cl· (21)

Cl· detection can be achieved via LFP by directly monitoring its characteristic absorption peak. As shown in Figure 3, CA generates transient absorption spectra upon photolysis, where both CH₃COCH₂· and Cl· are produced. Due to weak absorption of CH₃COCH₂· at 295 nm, it does not interfere with the spectroscopic detection of Cl·. In actual LFP operations, the absorption spectrum of the radical can be used to determine its maximum absorption peak, lifetime, and other key physicochemical parameters. According to Figure 3, the absorption band of Cl· ranges from 280 to 380 nm, with a maximum absorption at 320 nm (ε = 4500 M⁻¹·cm⁻¹). At 5 µs after photolysis, the absorbance at 320 nm drops to 5.0 × 10⁻³ M, indicating a very short lifetime. Furthermore, by using LFP to introduce reference compounds (RC), the characteristic behavior of Cl· can be verified. Typically, SCN⁻ is used as the RC, and the product (SCN)₂⁻ can be used to monitor Cl· concentration changes via the absorbance at 470 nm.

Figure 3: Transient absorption spectrum of a 10 mM chlorinated acetone working solution after photolysis. (Source: Xiao* et al., Water Research. 2023, 224:1, 120526).

For Cl₂⁻·, its maximum absorption peak appears at 340 nm as detected by LFP, with a molar extinction coefficient (ε) of 9600 M⁻¹·cm⁻¹ and a lifetime of approximately 50 μs, which is significantly higher than that of Cl· (see Figure 4). In practice, because Cl₂⁻· formation is typically accompanied by the generation of Cl·, and the absorption bands of Cl₂⁻· and Cl· significantly overlap at 320 nm, experiments generally increase the Cl⁻ concentration (greater than 1 mM) to suppress the interference from Cl·.

3. LFP Applications in Free Radical Mediated Reactions

3.1 Kinetics Measurement

(1) Self-Recombination

where:

• [R]₀: the initial concentration of the radical,

• [R]ₜ: the concentration of the radical at time t during the reaction.

According to the Beer-Lambert law, the value of [R]ₜ can be determined from the absorbance A: 

A = ε × [R]ₜ × l (24)

where:

• A: radical absorbance at time t,

• l: optical path length (usually 1 cm).

Figure 4: Transient absorption spectrum of the working solution containing 10 mM S₂O₈²⁻ and 0.5 M Cl⁻ at pH = 7.
(Source: Chu et al., Water Research, 2023, 224:1, 120526)

This method is only applicable to radical species with distinct characteristic absorption peaks. For example, SO₄⁻· exhibits a prominent characteristic absorption peak at 450 nm. By monitoring the time-dependent change in ΔA at 450 nm for SO₄⁻·, the self-recombination rate constant kₛᵣ can be determined as 1.2 × 10⁹ M⁻¹·s⁻¹ (Figure 5A). Similarly, in a solution with a Cl⁻ concentration of 0.1 M, the decay kinetics of Cl₂⁻· at 340 nm yield a kₛᵣ of 6.5 × 10⁸ M⁻¹·s⁻¹ (Figure 5B).

Figure 5 (A) Transient absorption spectroscopy kinetics of SO₄⁻· at 450 nm under different salicylic acid (SAL) concentration conditions (Source: Zhou et al., Water Research. 2017, 123, 715–723)

Figure 5 (B) Transient absorption spectrum of Cl₂⁻· at 340 nm in SAL solution (Source: George and Chovelon, Chemosphere, 2002, 47:4, 385–393).

(2) Reaction Kinetics Between Radicals and Target Pollutants

The method of using LFP to measure the degradation kinetics of target pollutants via radical intermediates can be divided into direct and indirect approaches. The direct method involves monitoring the degradation of the target compound (TC), the accumulation of products, or the decay of the radical to determine the rate constant k. Generally, the direct approach is the preferred method for obtaining kinetic parameters. However, it may not be effective when the radical's absorption in the UV-visible range (200–760 nm) is weak or when the absorption peaks of TC and its reaction products are unknown or overlapping. In such cases, an RC (reference compound) can be introduced for indirect measurements of the target reaction.

The direct method is suitable for radicals with distinct absorption peaks. When TC is present, changes in radical concentration result from both its self-decay and its reaction with TC. The concentration variation can be described as:

where:

• k′: pseudo-first-order reaction rate (s⁻¹) measured at different TC concentrations.

By plotting k′ values on the vertical axis and the initial TC concentrations on the horizontal axis, a straight line can be obtained. The slope of this line corresponds to the second-order reaction rate constant k between TC and the radical.

For example, in the reaction between SO₄⁻• and benzoic acid (a common scavenger), as shown in Figure 6, the LFP technique is used to directly monitor the absorbance changes of SO₄⁻• at 450 nm. By fitting the absorbance data at various benzoic acid concentrations, a set of k′ values is obtained. A linear relationship between k′ and the concentration of the scavenger is established, and the slope of the line, 6.3 × 10⁸ M⁻¹ s⁻¹, is determined to be the rate constant k for the reaction between SO₄⁻• and benzoic acid.

Figure 6: Linear relationship between the pseudo-first-order decay rate constant k′ of SO₄⁻• and the concentration of benzoic acid (a scavenger). (plot: transient absorption spectrum of SO₄⁻• at 450 nm under different benzoic acid concentrations.)
(Source: Caregnato et al., The Journal of Physical Chemistry A. 2008, 112(6), 1188–1194)

Product formation kinetics (product buildup kinetics, p.b.k.) is also a typical method of direct determination. This approach requires that the characteristic absorption peak of the product generated by the reaction between the free radical and the target compound (TC) is known, and that the absorption spectra of other free radicals, precursors, and TC in the system do not overlap. Once these conditions are met, one can monitor the change in ΔA at a specific absorption wavelength over time to determine the value of k. The kinetic expression for product formation is:

d[product]/dt = k[TC][R] (27)

Since TC is present in large excess compared to the free radical, the reaction between the free radical and TC follows pseudo-first-order kinetics. The first-order rate constant for product formation, kf, can be derived as:

[product]t=[R]0(1−e−kft) (28)

kf = k × [TC] (29)

The value of kf is linearly related to [TC], and the slope of this line corresponds to the value of k between the free radical and TC. For example, using DEET as a model compound and employing LFP under 355 nm laser excitation to generate HO• via HNO₂, the kinetics of the reaction between HO• and DEET can be studied. It is known that the transient product p-methylbenzyl radical generated by the reaction of HO• with DEET exhibits a distinct absorption at 330 nm. As shown in Figure 7, the growth curve at 330 nm yields a rate constant k for the HO• and DEET reaction of 4.95 × 10⁹ M⁻¹·s⁻¹.

Figure 7 (A) Kinetic growth curve of the product (p-ethylphenyl radical) from the reaction of HO· with different concentrations of DEET at 330 nm;
Figure 7 (B) Linear relationship between the apparent first-order growth rate constant kf′k_f' of p-ethylphenyl radical and [DEET] concentration.
(Source: Song et al., Water Research. 2009, 43:3, 635–642)

For free radicals that do not exhibit distinctive absorption in the UV-visible range, or when the absorption peaks of the target compound (TC) and its reaction products are unknown or overlapping, indirect methods can be used to determine the reaction rate constant kk between TC and the radical. The main idea is to add varying concentrations of TC into the system, where the presence of TC competes with a reference compound (RC) for reaction with the free radical, leading to different product formation rates. By varying TC concentrations and monitoring the changes in product absorbance, a functional relationship between product formation and TC concentration can be established to calculate the reaction rate constant kk. The derived equation is as follows:

where:

• A0/w refers to the absorbance change of the product formed by the reaction between the free radical and RC when TC is not present

• Aw refers to the absorbance change when TC is present

• kRC is the reaction rate constant between the free radical and RC.

Using A0/w divided by Aw as the y-axis and the initial concentration of TC as the x-axis, a straight line with a slope of 1 can be obtained. This slope equals kTC, and since both [RC] and kRC are known, kTC can be calculated.

As shown in Figure 8, under 266 nm laser excitation, LFP generates HO• from H2O2. The rate constant k for the reaction with syringic acid (Syr) can be determined. SCN⁻ is used as the reference compound (RC), and the absorbance decay of (SCN)2•⁻ at 460 nm is monitored under different concentrations of Syr. From the decay curves at various Syr concentrations, the rate constant k at pH 3 is found to be 5.0 × 1011 M⁻¹ s⁻¹. However, because Syr absorbs light itself and can interfere with accurate k determination by reducing radical formation, a correction is needed. If the uncorrected k exceeds 5.0 × 1011 M⁻¹ s⁻¹, it surpasses the diffusion-controlled rate for bimolecular reactions. The corrected formula for k is:

where:

• A(H2O2+SCN⁻) represents the absorbance of a solution containing H2O2 and SCN⁻ measured at a wavelength of 266 nm

• A(H2O2+SCN⁻+Syr) represents the absorbance of a solution containing H2O2, SCN⁻, and Syr measured at a wavelength of 266 nm.

• After correction, the rate constant k is 3.2 × 10¹⁰ M⁻¹·s⁻¹.

Figure 8 (A) Transient absorption spectra of (SCN)₂ •⁻ under different concentrations of Syr at 460 nm

Figure 8 (B) Pseudo-first-order decay rate constant k′ of (SCN)₂ •⁻ shows a linear relationship with [Syr].
(Source: Tong et al., Chemosphere. 2020, 256, 126997)

3.2 Mechanism Analysis

LFP can explore intermediates and products in reactions through spectral and kinetic modes, thereby inferring the reaction pathways and analyzing the mechanism of the reaction between free radicals and TC. In spectral mode, the absorption spectrum of TC's reaction with free radicals is scanned. Based on the absence of absorption peaks at certain wavelengths, intermediates can be identified, and the mechanism of product formation can be inferred. Taking the reaction between SO₄•⁻ and BPA as an example, its mechanism has long been debated: some studies suggest that the main pathway of the SO₄•⁻ and BPA reaction is radical addition formation (RAF), while others believe it proceeds via single electron transfer (SET), followed by hydrolysis of the intermediate, eventually causing ring-opening degradation. LFP’s spectral mode can clearly detect the characteristic absorption of this transient product, enabling accurate inference of the reaction mechanism. In the transient absorption spectrum of the working solution after excitation, a characteristic absorption of a BPA-SO₄•⁻ adduct was observed at 310 nm, indicating RAF as the dominant reaction pathway. Additionally, the kinetic mode of LFP can also infer the reaction mechanism. For example, by monitoring the decay kinetics of SO₄•⁻ at 450 nm, the reaction kinetics of SO₄•⁻ with a common coagulant, ferric chloride, were determined. The result showed a k value of 4.6 × 10⁸ M⁻¹·s⁻¹, suggesting that SO₄•⁻ undergoes hydrogen atom abstraction (HAA) with Fe³⁺, at a rate of approximately 10⁵–10⁷ M⁻¹·s⁻¹, indicating HAA may not be the dominant reaction pathway in this case.

For HO• radicals, the LFP system was used to detect the transient absorption spectrum of its reaction with diethyl phthalate (DEP). In spectral mode, a peak centered around 320 nm was observed, with its intensity decreasing over time. This absorption peak is attributed to the reaction product ‘DEP-OH’, suggesting that the radical addition formation (RAF) pathway is the main reaction route for HO• with DEP. Additionally, LFP technology can also be used to investigate the transfer processes of HO•-mediated reactions involving TrOCs (trace organic contaminants). Using 266 nm light to excite H₂O₂ to produce HO•, and studying its kinetics with bisphenol A (BP), the reaction rate constant k was determined to be 3.0 × 10⁸ M⁻¹·s⁻¹. Based on this, the intermediate product of the reaction can be detected via spectral mode, confirming RAF as the primary pathway of the reaction.

For Cl•-mediated reactions, LFP technology remains applicable. Taking the reaction between BPA and Cl• as an example, the characteristic absorption of the phenoxyl radical generated during the reaction appears at 390–420 nm. Since the phenoxyl radical is formed via hydrogen atom abstraction (HAA) between Cl• and phenol, this confirms the occurrence of an HAA pathway. In addition to HAA, the SET (single electron transfer) pathway of Cl• is also validated. For example, in the reaction between Cl• and 3,3',5,5'-tetramethylbenzidine (TMB), a distinct absorption peak at 580 nm was observed, which corresponds to the TMB•⁺ radical, suggesting the reaction proceeds via a SET pathway.

In actual operation, when multiple absorption peaks exist in a single spectrum, identifying the intermediate products can be very challenging. However, combining spectral mode with controlled experiments can effectively address this issue. For example, in the reaction between SO₄•⁻ and α,α,α-trifluorotoluene (TFT), significant absorption peaks were observed at both 330 nm and 400 nm. Some studies reported that the 330 nm absorption is attributed to SO₄•⁻ and the product radical of TFT formed through a single electron transfer, i.e., 1-chloro-2,4-dinitrobenzene radical (HCHD). This peak did not shift with changes in solvent oxygen concentration, indicating no structural change. In contrast, the 400 nm peak was less well understood. To determine whether this 400 nm peak is caused by HCHD, experiments were performed by varying the solvent oxygen concentration. It was found that the absorbance at this wavelength (ΔA₄) increased with oxygen concentration, suggesting that the 400 nm peak originates from a different intermediate than HCHD. By combining kinetic mode with spectral mode, one can also identify multiple intermediates associated with different absorption peaks. The process begins with spectral mode to obtain the transient absorption spectrum after the sample is irradiated, and identify the specific ΔA corresponding to each intermediate’s characteristic wavelength. Then, using kinetic mode, the time-dependent changes of ΔA at different wavelengths are tracked to determine the lifetimes of these intermediates. Ultimately, the differences in their lifetimes can help determine whether the species belong to the same substance. For instance, in the reaction between BPA and SO₄•⁻, the LFP spectral mode showed distinct characteristic absorption peaks at 410 nm and 310 nm. Following this, kinetic mode was used to monitor the decay of ΔA at these wavelengths. It was found that the decay rate at 410 nm was slower than that at 310 nm, indicating that these two wavelengths correspond to different intermediate species.

Conclusion

This article focused on LFP principles, setup, application modes, and its use in studying free radical mediated pollutant degradation. Compared to traditional flash photolysis, LFP offers high time resolution and is effective for measuring transient species and kinetic parameters. It supports mechanistic studies and may provide guidance for advanced education and pollution control research.

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